1. Field of the Invention
This invention is related to a rotational speed detection unit, specifically for use in detecting a rotational speed, for example, the rotational speed of the vehicle wheels, or of the main spindle of the machine tools.
2. Description of the Related Art
Rotational speed detection units involving a variety of mechanisms have heretofore been used to detect the rotational speed of a vehicle wheel to control an anti-lock braking system (ABS) or a traction control system (TCS), or to detect the rotational speed of a shaft such as the main spindle of a machine tool.
In general, rotational speed detection units for controlling an ABS or a TCS, are provided with a sensor wherein an emf induced in a coil changes in accordance with changes in magnetic flux density with rotation of the wheel. With such a unit however, the output of the sensor at low rotational speeds is small, and hence there is an unavoidable increase in cost on the controller side for accurate rotational speed detection to be achieved even at low outputs. Moreover, rotational speed detection at extremely low rotational speeds close to zero rpm is theoretically impossible.
As rotational speed detection units which give an output at a constant magnitude regardless of rotational speed, thus enabling rotational speed detection at extremely low speeds, there are the well known rotational speed detection units incorporating a Wiegand wire sensor or an amorphous magnetostriction wire sensor. A rotational speed detection unit incorporating a Wiegand wire sensor is disclosed for example in the American scientific magazine, "Electronics", Jul. 10, 1975, pp. 100-105, or in U.S. Pat. No. 3,820,090. Moreover, a rotational speed detection unit incorporating an amorphous magnetostriction wire sensor is disclosed for example in the Journal of the Japan Society of Applied Magnetism, Vol. 8, No. 2 (1984).
The Wiegand wire is a wire formed from a magnetic material, with the interior and the outer peripheral portion of the wire being different in coercivity or coercive force, while the amorphous magnetostriction wire is a wire wherein a residual stress is formed in the surface layer by rapid cooling of a molten metal.
Both the Wiegand wire and amorphous magnetostriction wire have the property that if placed in an external magnetic field parallel with the axis of the wires, then when the direction of the external magnetic field is changed, the direction of the internal magnetic flux reverses abruptly (instantaneously).
To detect this change in direction of flow of the magnetic flux, a coil is arranged around the Wiegand wire in the Wiegand wire sensor, while a coil is arranged around the amorphous magnetostriction wire in the amorphous magnetostriction wire sensor.
FIGS. 11 and 12 show two examples of well known constructions for rotational speed detection units which use such a Wiegand wire or amorphous magnetostriction wire sensor. Both of these rotational speed detection units incorporate a tone wheel 1 mounted on a rotation part, and a sensor 2 mounted on a non-rotation part.
The tone wheel 1 is made from a multipolar magnet, with South and North poles alternately disposed at even spacing.
The sensor 2, as shown in FIG. 13(A) and FIG. 13(B) (to be discussed hereunder), comprises a magnetic wire 3 (Wiegand wire or amorphous magnetostriction wire) and a coil 4 surrounding the magnetic wire 3.
With the construction of the first example shown in FIG. 11, the sensor 2 is radially disposed relative to the tone wheel 1, with one axial end thereof placed adjacent to an outer peripheral face of the tone wheel 1.
With the construction of the second example shown in FIG. 12, the sensor 2 is disposed parallel with the rotation axis of the tone wheel 1, with the whole of the sensor 2 placed adjacent to the outer peripheral face of the tone wheel 1.
When using a rotational speed detection unit with the tone wheel 1 and the sensor 2 arranged in this manner, the direction of the magnetic field in which the sensor 2 is placed changes alternately with rotation of the tone wheel 1.
For example with the arrangement of FIG. 11, when the sensor 2 is opposed to a North pole on the outer peripheral face of the tone wheel 1, a magnetic field is generated around the sensor 2 in the direction of the full line arrow of FIG. 13(A), and a magnetic flux flows in the magnetic wire 3 in the same direction indicated by the broken line arrow.
On the other hand, when the sensor 2 is opposed to a South pole on the outer peripheral face of the tone wheel 1, a magnetic field is generated around the sensor 2 in the direction of the full line arrow of FIG. 13(B), and a magnetic flux flows in the magnetic wire 3 in the same direction indicated by the broken line arrow.
Since this change (reversal) in flow direction of the magnetic flux in the magnetic wire 3 occurs instantaneously, a pulse-like signal voltage, as shown in FIG. 14, is induced in a coil 4 of the sensor 2. Hence, if this signal voltage is output to a controller, the rotational speed of the rotating part on which the tone wheel 1 is mounted can be determined. Moreover, since the abovementioned reversal is effected instantaneously irrespective of the rotational speed of the tone wheel 1, then a signal voltage of a constant magnitude is generated in the coil 4. Accordingly, the requirement for a circuit on the controller side to cope with the change in magnitude of the signal voltage is obviated, enabling a lower cost controller.
With the conventional rotational speed detection unit constructed and operated as described above, however, a stable output signal often cannot be obtained because an adequate magnetic field is not applied to the magnetic wire 3 of the sensor 2.
With the construction of the first example shown in FIG. 11, the magnetic field in the region of the one axial end of the sensor 2 is comparatively large, while the magnetic field in the region of the other axial end of the sensor 2 is small. This is because the magnetic field at the outer periphery of the tone wheel 1 drops rapidly with distance from the outer peripheral face of the tone wheel 1. Therefore with the construction as shown in FIG. 11 wherein sensor 2 is radially disposed relative to the tone wheel 1, as the gap between the tone wheel 1 and the one axial end of the sensor 2 is increased, the magnetic field at the other axial end of the sensor 2 remote from the tone wheel 1 is reduced, so that an even magnetic field is not applied to the whole of the sensor 2. As a result, the switching (reversal) of the direction of the magnetic flux in the magnetic wire 3 becomes unstable, resulting in an unstable output signal.
Moreover, with the second example shown in FIG. 12, since the North pole and the South pole are simply arranged alternately on the outer peripheral face of the tone wheel 1, then the component of the magnetic field in the axial direction of the sensor 2, due to the tone wheel 1, is small. Therefore, if the gap between the tone wheel 1 and the sensor 2 is increased, the switching of the direction of the magnetic flux in the magnetic wire 3 of the sensor 2 becomes unstable, again resulting in an unstable output signal.